1. Field of the Invention
The present invention relates to a multiple voltage generator, and more particularly, to a multiple voltage generator for driving a liquid crystal display (LCD) panel.
2. Discussion of the Related Art
Flat panel displays have been increasingly used in general computers and television sets as well as notebook computers. Among the flat panel displays, an LCD is widely used. The LCD display includes an LCD panel and a panel driver for driving the LCD panel. In driving the LCD panel, 2.sup.m voltage sources are necessary to display an m-bit picture signal. However, if the picture signal exceeds 5 bits, it is practically impossible to provide such a large number of the voltage sources corresponding to all the gray scale levels. Thus, various researches have been attempted to develop an alternative way to display the gray scale levels with fewer voltage sources.
One method is to create virtual intermediate levels. There have been proposed a frame rate control method and a dithering method. In the frame rate control method, an intermediate gray scale between two voltage levels is obtained by turning a pixel on and off during several frames. In the dithering method, an intermediate level is determined by using a mean value at which several pixels bound into one are turned on and off.
Another method of representing intermediate levels is to generate voltages corresponding to the intermediate levels. This includes an interpolation method and an intermediate voltage selection method. In the interpolation method, the desired mean value of a square wave, which is to be applied to a pixel, is obtained by adjusting the duty ratio of the square wave. In the intermediate voltage selection method, multiple voltage levels between two predetermined voltage levels are obtained by a voltage dividing resistor connected to the two voltage levels.
The interpolation method utilizes a SCOL circuit as shown in FIG. 1. Signals TM1, TM2, TM3 and TM4 are square waves whose duty ratios are 1:7, 2:6, 3:5 and 4:4, respectively. The TM waves of these signals are inverted to form signals whose duty ratios are 1:7, 2:6, 3:5, 4:4, 5:3, 5:2, 7:1 and 8:0. The upper 3 bits of the 6-bit video data signal VD select two voltage levels forming both ends of eight voltage level intervals, i.e., two of S0, S8, S16, S24, S32, S40, S48, S56, and S64. The lower 3 bits select one of eight TM waves to form 64 square waves. Thus, 64 voltage levels are applied to a pixel which can be modeled by a lowpass filter (LPF). If the frequency of the square wave is higher than the cut-off frequency of the LPF, only the mean value of the square waves is applied to the pixel, i.e., 64 voltage levels are applied thereto.
The intermediate voltage selection method utilizes a digital-to-analog converter circuit (DAC) as shown in FIG. 2. The upper 3 bits of a 6-bit video data are decoded in a decoder 20, and the decoded signal is transferred to the first and second voltage selectors 22 and 23. According to the decoded signal, the first voltage selector 22 selects one voltage level out of eight voltage levels V0 to V7, and the second voltage selector 23 selects another voltage out of eight voltage levels V1 to V8. The selected voltage levels are transferred to a voltage dividing resistor block (VDRB) 24. The VDRB 24 generates eight voltage levels between the two selected voltage levels, and the eight voltage levels are transferred to the third voltage selector 25. The lower 3 bits of the video data is decoded at a decoder 21, and the decoded signal is transferred to the third voltage selector 25. The third voltage selector 25 outputs one of the eight voltage levels according to the decoded signal. For example, when the video data is 100100, the first voltage selector 22 selects a voltage level V4, and the second voltage selector 23 selects a voltage level V5. The selected voltage levels V4 and V5 are transferred to the VDRB 24. The VDRB 24 generates eight voltage levels between the two voltages V5 and V4. A voltage signal having a magnitude of (V.sub.5 -V.sub.4).times.4/8 is selected by a third voltage selector 25. A signal of magnitude V.sub.4 +(V.sub.5 -V.sub.4).times.4/8 is then outputted. This way, 64 voltage levels are generated to display 64 gray scales.
The above-mentioned methods have the following drawbacks. The interpolation method requires a separate generator for generating a transverse magnetic (TM) wave. The intermediate selection method utilizes dividing resistors for generating voltages. Although the circuit is simple, the dividing resistors occupy a large area in the device. The conventional frame rate control method has a problem of flickering. In the dithering method, display resolution is sacrificed.